Method of determining trace amounts of gases

ABSTRACT

METHOD OF DETECTING TRACE AMOUNTS OF VOLATILE SUBSTANCES SUCH AS CARBIN OXIDES, NITROGEN OXIDES, SULFUROXIDES AND OXYGEN, COMPRISING TAKING A MEASURED SAMPLE OF THE GASEOUS MIXTURE CONTAINING THE GAS TO BE DETECTED, SEPARATING THE GAS BEING DETECTED FROM THE MIXTURE WHERE NECESSARY, FORMING A FIRST COMPONENT WHICH INCLUDES THE GAS BEING DETECTED AND, PREFERABLY, A CARRIER GAS, PROVIDING A SECOND COMPONENT WHICH CONTAINS A SUBSTANCE TO BE REACTED WITH THE FIRST COMPONENT TO PRODUCE CHEMIIONS, ACTIVATING AT LEAST ONE OF THE SUBSTANCES TO PRODUCE   THE SPECIES NECESSARY TO FORM CHEMI-IONS, MIXING THE COMPONENTS TO FORM THE CHEMI-IONS, PASSING THE MIXTURE BETWEEN ELECTRODES AND MEASURING AN ELECTRICAL CURRENT PRODUCED BY THE CHEMI-IONS FORMED.

Jan; 30, 1973 FONTIJN ETAL. 3,713,773

METHOD OF DETERMINING TRACE AMOUNTS OF GASES Filed Nov. 9. 1970 PUMP TOVACUUM ARTHUR FONTIJN BY PIETER H. VREE mew ATTORNEY United StatesPatent O 3,713,773 METHOD OF DETERMINING TRACE AMOUNTS OF GASES ArthurFontijn, Princeton, N.J., and Pieter H. Vree,

Croton-on-Hudson, N.Y., assignors to Aerochem Research Laboratories,Inc. Continuation-impart of application Ser. No. 663,369, Aug. 25, 1967.This application Nov. 9, 1970, Ser.

Int. Cl. G01n 27/70 US. Cl. 23-232 R 19 Claims ABSTRACT OF THEDISCLOSURE Method of detecting trace amounts of volatile substances suchas carbon oxides, nitrogen oxides, sulfur oxides and oxygen, comprisingtaking a measured sample of the gaseous mixture containing the gas to bedetected, separating the gas being detected from the mixture wherenecessary, forming a first component which includes the gas beingdetected and, preferably, a carrier gas, providing a second componentwhich contains a substance to be reacted with the first component toproduce chemiions, activating at least one of the substances to producethe species necessary to form chemi-ions, mixing the components to formthe chemi-ions, passing the mixture between electrodes and measuring anelectrical current produced by the chemi-ions formed.

CROSS-REFERENCE TO RELATED APPLICATIONS This application is acontinuation-in-part of our copending application Ser. No. 663,369,filed Aug. 25, 1967, and now US. Pat. No. 3,540,851 issued Nov. 17,1970.

BACKGROUND OF THE INVENTION Growing interest in the study and control ofair pollution has focused attention on methods of analyzing samples ofair for as little as one part per billion of various substances commonlypresent in air as impurities. These substances include such things aspollen, dust of various kinds, acids, alkalies and also gases and vaporsof various types such as hydrocarbon compounds, nitrogen oxides, carbonoxides, sulfur oxides and ozone.

There are satisfactory methods of analysis for very small amounts ofsome of the gases such as hydrocarbon compounds. But, in the case ofnitrogen oxides, carbon oxides, sulfur oxides and ozone, although thereare generally useful methods of analysis, these are not suited forrapidly and routinely detecting and quantitatively measuring quantitiesas low as, say, one part per billion.

One well known method of analyzing the contents of a gaseous mixture isto use a gas chromatographic column to separate the components of themixture and then to measure the amounts of each constituent with anapparatus such as a thermal conductivity cell, a thermistor cell, amicrocoulometric unit, or an argon ionization detector. These measuringmeans are not sensitive enough, however, to analyze the gases of mostinterest in this invention in the parts per billion range.

The helium ionization detector has been reported in the literature,however, which is said to be accurate in the parts per billion range,for detecting oxygen, argon, nitrogen, carbon monoxide, carbon dioxideand some other gases. This apparatus is not as adaptable to routineanalyses as desired and it has certain diificulties in its maintenance.Moreover, its operation is affected by the presence of certainimpurities present in the carrier gas. Recent reports indicate that somefurther analytical methods, specific for one or two oxides only, are nowPatented Jan. 30, 1973 ice OBJECTS OF THE INVENTION A principal objectof the present invention is to provide an improved method of detectingvolatile substances such as carbon oxides, nitrogen oxides, sulfuroxides and oxygen in a rapid and routine manner, with a sensitivity of afew parts per billion.

SUMMARY OF THE INVENTION The invention comprises an improved method ofdetermining trace amounts of a volatile substance such as an oxide ofcarbon, nitrogen or sulfur, or of oxygen alone, by the steps of:

(a) If the gas being determined is in a mixture in which interferingsubstances are present, first separating the gas to be determined fromthe mixture;

(b) forming a first component comprising a gaseous stream composed ofthe gas being detected, which may or may not be mixed with a carriergas;

(0) providing a second component which contains a substance to bereacted with the first component;

(d) activating at least one of the substances to produce the speciesnecessary to form chemi-ions;

(e) mixing the components so as to form the chemiions;

(f) measuring an electrical current produced by the chemi-ions formed.

The electrical current, calibrated against a standard, is directlyrelated to the concentration of the substance being detected.

The physico-chemical principle on which the method of the presentinvention is based is the production of chemi-ions in reactions ofactivated species. By activated species is meant a substance to whichadditional energy has been imparted to convert it to a more reactive,gaseous form. Typical activated species are oxygen atoms, nitrogenatoms, hydrogen atoms, halogen atoms, hydroxyl radicals, excitednitrogen molecules, excited nitrogen atoms, excited oxygen atoms andozone. The volatile substance to be detected and/or the substance in thesecond component may be activated, i.e. converted at least partiallyinto activated species, by any one of several known methods. Forexample, they can be activated by electrical discharge methods, thermalmethods, or photochemical methods, as described, for example, in anarticle The Production, Detection and Estimation of Atoms in the GasPhase by K. R. Jennings, Quarterly Reviews, vol. 15, p. 237 (1961).

The method of this invention can conveniently be used to analyze theoutput of a gas chromatography, but can equally as well be used in acontinuous analyzer of a substance to be detected. The preferredembodiment includes a pair of electrodes placed in the gas flowdownstream from an activator. Between the discharge and the electrodesthe gas to be analyzed flows into a second gas stream, thus producingchemi-ions. In some cases, the gas to be analyzed will directly form theproducts which are needed to produce the ions; in other cases the gas tobe analyzed must be activated to produce the necessary ion precursors.

Although some embodiments of the present method employ an electricaldischarge, they are not discharge detectors. In discharge detectors achange in electrical properties between the electrodes maintaining thedischarge is observed when the sample passes through. In the presentafterglow ionization detection method, the non-ionic, long-liveddischarge products are used to produce ions downstream from thedischarge, where the discharge ionization is no longer detectable.

DESCRIPTION OF THE DRAWINGS The single figure is a schematic diagram ofan apparatus suitable for carrying out the method of the invention.

Referring now to the drawing, the apparatus comprises a chromatographicgas separation column 2 of conventional type, having an inlet tube 4connected to one end and an outlet tube 6 connected to the opposite end.A sample-injecting inlet port 8 is connected to the inlet tube 4. Theoutlet tube 6 leads into the measuring apparatus which will now bedescribed.

The measuring apparatus includes a discharge gas inlet tube 10 passingthrough a microwave cavity 12, of conventional design, to a Y-shaped gasdivider unit 14. One leg of the Y comprises a sample arm tube 16containing a pair of electrodes 18 and 20. The other leg of the Ycomprises a reference arm tube 22, also containing a pair of electrodes24 and 26. Both the sample arm tube 16 and the reference arm tube 22 arejoined at their outlet ends to a line 28 leading to a vacuum pump (notshown).

Connected into the sample arm tube 16 is a continuation of theoutlettube 6 from the gas chromatograph column 2. Connected into the referencearm tube 22 is a tube 30 from a source of ballast gas (not shown). Thislast-mentioned tube 30 first passes through the microwave cavity 12 asdoes the tube 6.

Also connected to the gas discharge inlet tube 10 may be another tube 32for introducing a gaseous catalyst in a modification of the invention.

One of the electrodes 18 in the sample arm 16 is connected by a lead 34to a current-measuring device 36 such as a vibrating reed electrometer.The other electrode in the sample arm 16 is connected to one side, e.g.,the positive side, of a battery 38 by a lead 39. One of the electrodes24, in the reference arm 22, is also connected to the current-measuringdevice 36 by a lead 40. The other electrode 2.6, in the reference arm22, is connected to the same side of the battery 38 as is electrode 20by a lead 42. The other (negative) side of the battery 38 is connectedto ground. The current-measuring device 36 also has a terminal 44connected to ground.

EXAMPLES The following is a first example of how to use the apparatus incarrying out the method of the invention in detecting nitric oxide (NO).First, a flow of carrier gas, e.g., helium, is started through the inlettube 4 and through the chromatograph column 2 to establish equilibriumconditions. At about the same time, a flow of ballast gas, also helium,is started through the ballast gas inlet tube 30 and into the referencearm 22 of the measuring apparatus, and a mixture of nitrogen and heliumcontaining about one volume percent nitrogen is passed into thedischarge gas tube 10. Meanwhile, the vacuum pump is also started toestablish a vacuum of about 1 mm. of Hg in the sample arm 16 and thereference arm .22. Then the microwave cavity 12 is energized. Thiscauses the gaseous molecules to be broken down into atoms:

The flow rate of the nitrogen-helium mixture is about 1 cc./second.

A 1 cc. sample of the gas mixture which contains the NO to be measuredis then injected into the sample injection port 8 and permitted to flowthrough the gas chromatograph column 2. It is assumed that previous testruns have been made to determine the exact time interval it will takefor the NO to appear at the exit end of the column. In the apparatusshown, the sample is passed through the microwave cavity to produce theoxygen atoms needed for the production of ions. However NO does not needto be activated in order to obtain a measurement of its concentration(see Example No. 2, below).

The current measurement apparatus 36 includes a conventionalelectrometer connected to a recorder. When the atoms from the N0 of theinjected sample pass through the sample arm 16, a peak will be observedin the recordedreading of the electrometer. The reason for theappearance of the peak is that in mixtures of nitrogen atoms and oxygenatoms, ions are produced by reactions involving excited N and/or excitedN0 molecules in reactions such as:

in which N and NO* are excited molecules formed by NN and N-Oatom-association reactions.

The area under the peak of the recorded curve is meas ured to give arelative measurement of the concentration of NO in the sample. To get anactual reading of concentration, this value must be compared with thearea under a peak recorded with a known sample of NO. The relativevalues are different for each apparatus set-up.

The method of this example can also be used to detect any other gasthat, when subjected to an electrical discharge, produces oxygen atoms.Examples of such other gases are: sulfur oxides, carbon oxides, nitrogenoxides, molecular oxygen, and other volatile oxides. All of these gasesmay be activated by electrical discharge methods according to a varietyof known techniques. Some of them may also be activated by othermethods. For example, oxygen atoms can be produced by passing molecularoxygen over a hot rhenium Wire or a Nernst glower. Photochemical methodsare also suitable for producing atoms.

In general, the methods of the invention can be used to detect othersubstances which are not gaseous at ordinary room temperature if thetemperature is raised sufiiciently to vaporize them. Examples of suchsubstances are AS203, P203 and P205.

A second example of how to carry out the method of the inventioninvolves activating only the substance in the second component. Thevolatile substance being measured is not activated. In this embodimentof the method, nitric oxide can be detected as follows.

A measured sample of a sample stream containing NO is obtained as in theprevious example. Also, as in the first example, a mixture of nitrogenand helium containing about one volume percent of nitrogen is passedthrough the energized microwave discharge cavity (or other activator) toproduce nitrogen atoms. The nitrogen atoms and the nitric oxide of thesample stream react as follows:

and the nitrogen atom and oxygen atom mixture thus formed produce ionsin the same manner as in the first example.

It will be noted that, in this example, the sample being detected reactswith the second component to produce the ion precursors and thechemi-ions. The method of this example can be used to detect anyvolatile substance that reacts with nitrogen atoms to produce oxygenatoms. Examples of such substances are nitric oxide, nitrogen dioxideand ozone.

A third example of the method involves detecting a sample of a substancethat reacts directly with the second component to produce chemi-ions. Inthis example, a sample of cesium vapor, which may or may not be carriedin an inert carrier gas, such as helium, is reacted with a stream ofnitrogen gas that has previously been activated. The results may berepresented by reactions such as:

N is anexcited molecule formed by atom-association reaction or directlyfrom N in the activator. Other substances that can be detected by themethod of this example are those substances that can react directly withactivated nitrogen to produce ions; for example, lead, barium and sodiumatoms.

In Example 4 of the method, a different second component is illustrated.Here, the sample being detected, which may be carbon dioxide, forexample, may be passed through a gas chromatographic column, as inExample 1, and then passed through an activator to produce a reactionthat may be represented as:

and the products reacted with the second component, which in this caseis acetylene (C H which may or may not be mixed with an inert carriergas, such as helium, in about the same volume proportions as thenitrogen and helium of Example 1. The reaction which occurs is believedto be:

As discussed in the first example, any substance that produces oxygenatoms in an activator, can be used in this example. Examples of specificsubstances that can be determined are the same as in the first example.

In the method as described in Examples 1 and 2, the limit ofdetectability can be increased by the use of an additive that is capableof reacting with nitrogen atoms to form CN. Examples of such additivesare C N C F C H C H or other small organic molecules that contain one ormore unsaturated carbon to carbon bonds.

We claim:

1. A method of determining trace amounts of a volatile substancecomprising:

(a) providing a sample which includes said substance in a gaseous form;

(b) forming a first component comprising a gaseous stream including saidsample, which component may or may not include a carrier gas;

() providing a second component which contains a substance to be reactedwith said first component;

(d) activating at least one of said substances;

(e) mixing said components downstream from the activating means to formchemi-ions; and

(f) measuring an electrical current produced by said chemi-ions as ameasure of the amount of said volatile substance.

2. A method according to claim 1 in which said first component includesa carrier gas.

3. A method according to claim 1 in which only said substance in saidsecond component is activated.

4. A method according to claim 3 in which the substance being determinedis nitric oxide.

5. A method according to claim 3 in which the substance being determinedis nitrogen dioxide.

6. A method according to claim 3 in which the substance being determinedis ozone.

7. A method according to claim 3 in which said second componentcomprises nitrogen and an additive that reacts with nitrogen atoms toform CN.

8. A method according to claim 1 in which only said volatile substancein said first component is activated.

9. A method according to claim 8 in which the substance being determinedis a carbon oxide.

10. A method according to claim 8 in which the substance beingdetermined is an oxide of sulfur.

11. A method according to claim 8 in which the substance beingdetermined is an oxide of nitrogen.

12. A method according to claim 8 in which the substance beingdetermined is molecular oxygen.

13. A method according to claim 1 in which both of said substances areactivated.

14. A method according to claim 13 in which the substance beingdetermined is a carbon oxide.

15. A method according to claim 13 in which the substance beingdetermined is an oxide of sulfur.

16. A method according to claim 13 in which the substance beingdetermined is an oxide of nitrogen.

17. A method according to claim 13 in which the substance beingdetermined is molecular oxygen.

18. A method according to claim 13 in which said second componentcomprises nitrogen and an additive that reacts with nitrogen atoms toform CN.

19. A method of measuring the percentage of nitric oxide in a gaseousmixture comprising:

(a) taking a small measured sample of said gaseous mixture;

(b) separating the nitric oxide being measured from said sample;

(c) forming a first component which consists essentially of saidseparated nitric oxide and an inert carrier gas;

(d) forming a second component which consists essentially of nitrogenand an inert carrier gas;

(e) activating said nitrogen;

(f) then mixing said first and second components whereby said activatednitrogen reacts with said nitric oxide to produce chemi-ions, and

(g) measuring an electrical current produced by said ions as a measureof the amount of said nitric oxide present in said gaseous mixture.

References Cited Herron et al.: J. Chem. Phys. 30, No. 4, April 1959,pp. 879-885.

MORRIS O. WOLK, Primary Examiner R. M. REESE, Assistant Examiner US. Cl.X.R.

23-254 R; 204-312; 25083.6 FT; 324-33

